A photonic crystal is a periodically structured composite material, with a unit cell whose dimensions are on the order of a wavelength of visible to infrared light. Such dimensions may broadly range from about 50 nanometers (nm) to about 10 micrometers  (μm), and preferably are smaller than one μm. These dimensions may be suitable, for example, to generate a photonic band gap useful with wavelengths between about 600 nm and about 1.65 μm, and more particularly, for example, wavelengths such as 1.3 μM and 1.55 μm as typically used in optical communications. Such a crystal is made from two constituent materials whose refractive indices greatly differ, such that the contrast ratio between them generally is at least 2:1. Three-dimensional (3D) photonic crystals typically consist of interpenetrating networks of dielectric material and air, the latter serving as the material of relatively low refractive index.
The defining characteristic of a photonic crystal is a range of frequencies within which no propagating electromagnetic modes exist. Multiple interference between waves scattered from each unit cell of the crystalline structure can open a photonic band gap. A photonic band gap is a range of frequencies, analogous to the electronic band gap of a semiconductor, within which no propagating electromagnetic modes exist. Structural defects in a photonic crystal may give rise to spatially localized electromagnetic modes, or microcavity-confined modes, at energies within the gap. Waveguides are formed by coupling such defects together. A waveguide operating at a frequency within a photonic band gap cannot leak, because there are no propagating electromagnetic modes in the surrounding photonic crystal capable of carrying energy away. In principle, this absence of leakage allows the fabrication of waveguides that turn corners in a distance on the order of the optical wavelength, requiring two orders of magnitude less space than semiconductor ridge waveguides currently used in integrated optics, which typically have a minimum bend radius greater than 100 μm.
Photonic band gap crystals can form the basis for miniaturized integrated optical circuits with length scales comparable with those of integrated electronics. Such crystals can serve, for example, as waveguides, splitters, optical insulators, optical filters, microcavity lasers, optical switches, routers, and in other photonic band gap applications. They can be designed to optically act on either a one dimensional (1D), two dimensional (2D) or 3D level. Such engineering applications require fabrication technologies for the cheap and rapid production of periodic structures that have the potential to incorporate engineered structural defects to create microcavities and waveguides. Other applications for these structures include uses as filters, catalysts, and biocompatible materials.
Despite the broad potential utility of materials having interconnected porous structures having sub-micron periodicity and high refractive index contrasts, conventional technologies are found difficult to provide cost effective methods of making them. According to one conventional method referred to as “log-piling,” layers of uniformly spaced grid elements formed, of a high refractive index material are painstakingly stacked together by serial lithography and etching. Although this method can produce a functional photonic band gap structure, such processing is labor and time intensive and therefore impractical for commercialization. Moreover, sufficiently accurate mutual registration of the layers is difficult to achieve, and control of the contours of the grid elements is limited. A variation of this method, involving fusion of successive prefabricated grid wafers, generates similar problems. Another tedious method for fabricating band gap structures involves drilling holes in a solid block of high refractive index material, by using a laser for example.
Further methods used to make crystalline structures having interconnected macroscopic porosity have involved creating a negative template mold into which a material having a high refractive index, or its precursor, is then filled. The negative template mold is then decomposed, for example by oxidation, to produce a positive final porous structure with high index contrast. These methods include various processes involving exposure of a material to a light pattern with light and dark regions, producing a void-filled structure. The resulting structures are referred to as positive tone structures if the voids occupy regions that were exposed to light regions of the light pattern. The resulting structures are referred to as negative tone structures if the voids occupy regions that were dark regions of the light pattern.
One group of methods used to make negative template molds for crystalline structures having macroscopic porosity has depended on chemical self-assembly techniques. For example, one of these methods involves self-assembly of colloids by sedimentation, forming a face-centered cubic lattice. Drawbacks to such a methodology include the prevalence of undesired lattice defects such as stacking faults, and the inability to obtain lattice types other than face-centered cubes and to otherwise control lattice parameters other than the colloid cell diameter. Another chemical self-assembly method involves selective decomposition of one block in a block copolymer to leave controlled porosity after processing. Cylinder and gyroid lattices can be produced by this method. However, lattice defects are prevalent, and pore size typically is less than 100 nm. Two-photon polymerization can be used to write periodic structures with different lattice constants. However, it is a rather slow point-wise writing process.
Holographic lithography is a method that has been successfully used to make a polymeric negative template mold suitable for producing a crystalline structure having interconnected macroscopic porosity. According to this method, a photosensitive material, for example, a film of a desired thickness, is subjected to an optical interference pattern resulting from multiple beam interference. The material can then be selectively polymerized or deprotected in regions where the film is exposed to the interfered optical signals. After subsequent development using a suitable solvent, a porous 2D or 3D template is obtained. Among the advantages of holographic lithography are an ability to select the porous structure's lattice constants, an ability to produce crystalline structures free of unintended defects, and the availability of inexpensive commercial means for implementation.
However, efforts to effectively use such a negative template polymeric mold to produce a positive final crystalline structure having a high refractive index contrast have not been entirely successful. Titanium tetraethoxide has, for example, been filled into such molds in sol-gel form. However, complete filling of such a mold may be problematic due to pore-clogging. Moreover, the refractive index of titanium dioxide upon decomposition of the negative- and positive-template organic components is only 2.0 to 2.4 on a scale in which air has a refractive index of 1.0, which is near the low end of about 2.0 for photonic band gap utility. In a related method, materials having refractive indices of less than 3, such as cadmium sulfide and cadmium selenide, have been electroplated into a polymer mold. However, the range of compounds suitable for electroplating is limited.
High temperature methods involving gas phase deposition, such as chemical vapor deposition (CVD), would be effective for filling a material of high refractive index, such as elemental silicon, into a mold. However, a polymer mold such as those discussed above clearly cannot withstand the temperatures of 400° C. or more, and often 500° C. or more, necessary to create the chemical vapor. In one effort to resolve this problem, sea urchin skeletons were used as negative template molds and filled with polydimethylsiloxane (PDMS) oligomers, which can be polymerized to solid form and then oxidized to silicon dioxide glass. However, this method necessarily depends on the irregular structure of a natural sea urchin skeleton to determine the mold structure, allowing no control over the uniformity, size, contours or interconnectivity of the pores. In general, the porosity resulting from such sea urchin based methods is on the order of about 20 to about 100 μm, far above the micron range. Moreover, photonic band gap structures require introduction of precisely positioned point defects to provide waveguide pathways through the otherwise non-propagating material, the introduction of which cannot be controlled by using sea urchin skeletons as negative template molds.
Accordingly, there is a need for a process for the production of negative template porous molds that can be used to produce positive final crystalline structures fabricated from materials having a high refractive index and having interconnected macroscopic pores.